1. Field of the Invention
The invention relates to a system and apparatus for the conversion of solar energy into microwave energy and transmission of such microwave energy through space to a receiving station.
2. Description of the Prior Art
Electrical power has become such an accepted form of energy that the demands have increased at such a rapid rate requiring the exploration for new sources. Unfortunately, present methods of generating electrical power pollute the environment and consume natural resources at a prodigious rate. New technologies have arisen with respect to the transformation of the solar energy directly into electrical power without the necessity of a thermal cycle or any moving parts and with potentially high reliability and long life. Another new technology involves the space program which permits the orbiting of large solar arrays to be illuminated by the sun. The dc power derived from the solar cells can be converted into microwave power and large amounts of such microwave power may be beamed efficiently over long distances of free space by means of a microwave beam. The carefully controlled microwave beam is captured and the microwave energy is rectified at the receiving station. The microwave beam transfer efficiencies run as high as 90 to 95%. To transmit the energy through the earth's atmosphere with relatively small attenuation due to moisture the use of a wavelength of approximately ten centimeters is desirable. The size of the microwave beam apertures necessary for efficient transmission is sufficiently large so that very high power handling capabilities is possible. Passive radiation of waste heat in the microwave energy transmitters is made possible by the large antenna array area and the use of highly efficient microwave generators. For the purposes of the specification the term "micorwaves" refers to energy in that portion of the electromagnetic energy spectrum having wavelengths in the order of 1 meter to 1 millimeter and frequencies in excess of 300 MHz.
The conversion of microwave energy to electrical power has been disclosed in prior art embodiments utilizing rectifier antenna (rectenna) arrays comprising a considerably large number of solid state semiconductor devices such as Schottky-barrier diodes together with half-wave dipoles each terminated in a semi-conductor diode means. A complete rectenna array is disclosed in U.S. Pat. No. 3,434,678 issued Mar. 25, 1969 to W. C. Brown et al and assigned to the assignee of the present invention. The wireless transmission of the microwave energy via beams is preferred due to the relatively high power density which can be focused upon a relatively small area at the receiving location. In accordance with the laws of optics the sharpness of the microwave beam varies as the ratio of antenna dimensions to the wavelength of the transmitted energy. Accordingly, for a given predetermined power density and beam sharpness very short wavelengths of microwave energy facilitates a corresponding decrease in the dimensions of the antenna means for transmitting and receiving. In addition, in long wave or radio-type transmission, difficulties are encountered as a result of natural or man-made interference and noise and this problem is considerably reduced with the transmission of microwave energy. The recent United States space program has demonstrated that the employment of very short microwaves for beamed energy is preferable for penetration of certain atmospheric reflection layers together with the advantages of accurate focusing and high power density of the transmitted energy.
Further examples of the utilization of microwave energy in space applications may be found in U.S. Pat. No. 3,174,705, issued Mar. 23, 1965 to D. Schiff et al, as well as 3,083,528, issued Apr. 2, 1963 and 3,114,517, issued Dec. 17, 1963, both to W. C. Brown. The foregoing patents are also assigned to the assignee of the invention and disclose various conversion means utilizing heat exchangers for conversion of mircowave energy into electrical energy or heat which can be utilized for directly or indirectly propelling a space station.
An example of the satellite solar power station concept is found in U.S. Pat. No. 3,781,647, issued Dec. 25, 1973 to P. E. Glaser. The satellite system includes several geostationary satellites positioned in an energy-receiving relationship to the sun. The solar energy conversion means include guidance and control means to correctly orient the energy conversion means to the source. The large solar cell array is located in an equatorial synchronous orbit around the earth where it is illuminated by the sun with a duty cycle in excess of 99%. The dc power derived from the solar cells is converted into microwave power and beamed to earth.
A most efficient superpower high frequency microwave energy generator operative in the desired band is the Amplitron which has a broad bandwidth and very high efficiency with extremely long life. The electron tube comprises a nonreentrant, non-resonant slow wave propagating structure having two terminations and a continuous cathode capable of emitting electrons from a major portion thereof. Cold cathodes having a high secondary electron emission ratio are particularly desirable in view of the absence of any requirements for power supplies for directly heating oxide-type cathode emitters. The illustrative superpower devices are capable of producing 15 to 20 kilowatts of average power at the desired frequency with capabilities in the region of 500 kilowatts or more of average power and 50 megawatts peak power. A complete description of the Amplitron microwave amplifier devices may be found in U.S. Pat. No. 2,673,306, issued Mar. 23, 1954, 2,859,411, issued Nov. 4, 1958, 2,933,723, issued Apr. 19, 1960 and 2,977,502, issued Mar. 28, 1961 all in the name of W. C. Brown and assigned to the assignee of the present invention. An example of a secondary emissive cold cathode structure is found in U.S. Pat. No. 3,096,457 issued July 2, 1963 to W. A. Smith, Jr. et al.
The current energy crisis throughout the world due to the growing shortage of the world supplies of fossil fuels, such as coal, oil and natural gas due to the increases in the level of consumption has made the search for new sources of energy imperative. The use of solar energy has now been proven in the space program and satellite solar power stations can be developed, hopefully, before the turn of the century. Solar energy offers an almost unlimited source for man's energy needs and the devices for conversion of the dc energy to microwave energy have expected efficiencies of approximately 85 to 90%. The direct coupling of the microwave energy sources to the transmitting phased antenna array should also result in expected transmission efficiencies in the 85 to 95% region. In acordance with the teachings of the referenced satellite solar power station U.S. Pat. No. 3,781,647 the actual size of the solar cell area for collection and conversion of the solar energy would be approximately 25 square miles. The transmitting phased antenna array would be approximately 0.6 of a mile in diameter, assuming a transmitting wavelength of approximately 10 centimeters. A transmitting antenna, illustratively, is designed to provide approximately 10,000 megawatts of beam power which could be supplied by 10,000 microwave energy generators, in 100 rows and 100 panels in each row with each tube having a 1 megawatt output. The present invention achieves the unique arrangement for an active phased antenna array with mircowave generators to provide for the maximum efficiency of the solar power station. Eventually solar power stations having a 200 square mile area would require one to two million microwave tubes for each station.